The present invention relates to a fuel cell that is useful for domestic cogeneration systems and portable power sources, particularly to a polymer electrolyte fuel cell in which a polymer is employed as an electrolyte.
The fuel cell generates electricity and heat simultaneously by electrochemically reacting a fuel such as hydrogen and an oxidant gas such as air at gas diffusion electrodes, and there are several kinds of fuel cells that employ different electrolytes and operating temperatures. Among these fuel cells, the polymer electrolyte fuel cell dominantly uses, as a polymer electrolyte, fluorocarbon polymer with a sulfonic acid group introduced as a side chain terminal group. An electrode reaction layer mainly composed of a carbon powder carrying a platinum group metal catalyst is formed on each side of an electrolyte membrane composed of the above-mentioned material in such a manner as to closely adhere to the electrolyte membrane. Further, a pair of conductive porous sheet materials, having both gas permeability and electric conductivity, is formed on outer surfaces of the electrode reaction layers in such a manner as to closely adhere thereto. The conductive porous sheet material and the electrode reaction layer constitute a gas diffusion electrode.
Disposed outside the gas diffusion electrodes are electrically conductive separator plates for mechanically securing the assembly of the electrodes and the electrolyte membrane and connecting adjacent assemblies electrically in series. A portion of the separator plate to come in contact with the electrode is provided with a gas flow path for supplying a reaction gas to the electrode surface and removing a generated gas and an excess gas. Gas sealing materials such as gasket or sealant are arranged on peripheral portions of the electrodes and the separator plates having a gas flow path, preventing two kinds of reaction gases from mixing together or leaking to outside.
As an ordinary power source, the fuel cell is configured to have a stacked structure, i.e., as a fuel cell stack where a plurality of unit cells, each comprising an electrolyte membrane, electrode reaction layers and separator plates, are stacked and a fuel gas such as hydrogen and an oxidant gas such as air are supplied to respective gas flow paths through manifolds from outside. Current generated at the electrode reaction layers is collected by the conductive porous sheet materials, passed through the separator plates, and taken to outside. The separator plate is often composed of a carbon material having electric conductivity, gas tightness and corrosion resistance, but a metallic separator made of stainless steel or other metals is also used since this separator has good forming workability, is low-cost, and can be made thinner.
The fuel cell stack is configured to circulate cooling water or antifreeze inside the cell stack in order to control the cell temperature, since heat is also generated during power generation utilizing the electrochemical reaction. It is common that the cooling water, when heated by this heat, is cooled by a heat exchanger which is disposed outside the cell stack such that it is again circulated therein.
In the fuel cell stack, the cooling water passes through a manifold for cooling water from the heat exchanger disposed outside, and then flows through a cooling unit having a flow path of the cooling water provided every 1 to several unit cells, thereby cooling the unit cells. The cooling water then passes through another manifold and returns to the heat exchanger. In the fuel cell stack of the conventional construction, materials of the cooling water circulation path corrode during operation of the cell stack, causing deterioration of cell performance and a gas leak. Also, corrosion causes dissolution of ions into the cooling water out of these materials and the ionic conductivity of the cooling water is thereby raised, so that there has also been a problem with respect to safety against a leakage of current in case of gas evolution in the cooling water and a water leak.
Further, in order to maintain a high ion-conductivity of the electrolyte, the conventional fuel cell stack needs to reduce contaminant ions from outside, particularly the concentration of ions dissolved in steam and humidifying water included in reaction gases. Thus, it needs addition of pure water to the humidifying water or replacement of the humidifying water.
In view of the foregoing, an object of the present invention is to provide a fuel cell system that is free from troubles caused by impurity ions by suppressing the corrosion of a portion of the fuel cell system to come in contact with cooling water and reducing the concentration of impurity ions in the cooling water.
Another object of the present invention is to provide a fuel cell system that is capable of maintaining the ionic conductivity of the electrolyte at a high level by reducing the concentration of ions dissolved in steam and humidifying water included in reaction gases.
In the present invention, a material adsorbing or absorbing impurity ions in cooling water is provided on a cooling water circulation path in order to suppress the corrosion of members of the cell system and prevent the concentration of impurity ions from rising. This can eliminate the problems that the cell performance is lowered by a current flowing in the cooling water and the safety is not secured in case of a water leak.
The present invention provides a fuel cell system comprising a fuel cell stack and a means for controlling the cell temperature by circulating a liquid coolant in the fuel cell stack or bringing it in contact with the fuel cell stack, the fuel cell stack comprising a plurality of unit cells that are laid one upon another, each of the unit cells comprising a hydrogen ion-conductive electrolyte membrane, a pair of gas diffusion electrodes which sandwich the electrolyte membrane, an anode-side conductive separator plate having a gas flow path for supplying a fuel gas to one of the electrodes, and a cathode-side conductive separator plate having a gas flow path for supplying an oxidant gas to the other of the electrodes, wherein a material adsorbing or absorbing ions is provided on a portion of the fuel cell system to come in contact with the liquid coolant.
It is preferable that the material adsorbing or absorbing ions adsorbs or absorbs ions at a speed or in an amount which is dependent on a potential difference between the liquid coolant and the material.
It is preferable that the material adsorbing or absorbing ions is electrically connected to the fuel cell stack and has a potential which is dependent on a potential of the connected portion of the fuel cell stack with respect to the liquid coolant, i.e., a potential that is almost equal or proportional to a potential of the connected portion of the fuel cell stack with respect to the liquid coolant.
It is preferable that the material adsorbing or absorbing ions comprises an intercalation material composed of carbon or a metal oxide.
As the liquid coolant, water or an organic liquid having a hydroxyl group is preferably used.
It is also possible to further comprise a means which is disposed on a flow path of the liquid coolant for intermittently applying a potential to the material adsorbing or absorbing ions.
The present invention also provides a fuel cell system comprising a fuel cell stack and a humidifying means for humidifying a reaction gas with steam, the fuel cell stack comprising a plurality of unit cells that are laid one upon another, each of the unit cells comprising a hydrogen ion-conductive electrolyte membrane, a pair of gas diffusion electrodes which sandwich the electrolyte membrane, an anode-side conductive separator plate having a gas flow path for supplying a fuel gas to one of the electrodes, and a cathode-side conductive separator plate having a gas flow path for supplying an oxidant gas to the other of the electrodes, wherein a material adsorbing or absorbing ions is disposed so as to be in contact with humidifying water of the humidifying means with a potential applied by the fuel cell stack.
While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.